Heretofore, in automotive vehicles and the like, with a view to improving fuel economy performance and/or exhaust gas purification performance, a technique of early warming up an engine during cold operation of the engine has been employed.
For example, JP 2010-163920A discloses a technique of achieving early completion of engine warm-up by, during cold operation of an engine, passing a small amount of coolant through a cylinder head in a direction from one end to the other end of a cylinder row (row of cylinders), while cutting off a coolant flow to a cylinder block. Then, along with a rise in temperature of coolant, coolant is additionally passed through the cylinder block in the direction from the one end to the other end of the cylinder row, and a flow rate of coolant to be circulated through the cylinder head is increased.
However, considering realization of further improved fuel economy, the above conventional technique is not enough in terms of promotion (speeding-up) of engine warm-up.
Therefore, it is conceivable that heat of exhaust gas flowing through an exhaust passage is utilized to allow coolant to rise in temperature, thereby promoting engine warm-up. In this case, however, the coolant draws heat of the exhaust gas to cause the exhaust gas to decrease in temperature, so that, in a catalyst of an exhaust gas purification device to be activated by means of heat of the exhaust gas, a period of time required to reach an activation temperature is increased, which is disadvantageous in terms of early activation of the exhaust gas purification device.
Meanwhile, primarily in order to reduce nitrogen oxides (NOx) in exhaust gas and improve fuel economy during partial-load conditions, a technique of extracting a part of post-combustion exhaust gas and leading the extracted exhaust gas to an intake side to allow it to be re-charged, so-called “EGR (Exhaust Gas Recirculation)”, has heretofore been performed.
In this technique, it is advantageous, particularly for diesel engines, to perform heat exchange between exhaust gas and coolant through an EGR cooler to thereby cool the exhaust gas, because, when a combustion temperature in a combustion chamber exceeds a given value, an amount of nitrogen oxides is sharply increased.
However, in a conventional system, during cold operation of the engine, coolant stagnates in the EGR cooler without flowing therethrough, so that the coolant is gradually raised in temperature due to heat of exhaust gas, and eventually boiled, which is likely to lead to breakage of the EGR cooler.